While you're busy stressing over that one gray hair, you may miss some of the subtle shifts that make getting older a good thing. It's time to celebrate!

Fewer pesky illnesses

Fewer pesky illnesses

You know how the flu often hits the youngest and oldest people the hardest? Your immune system will eventually weaken, but experts say you experience a sweet spot for fighting off infections in your thirties and forties. "Over time, we store more and more different kinds of antibodies in what I like to call our 'antibody libraries,'" says Roxanne Sukol, M.D., staff physician at the Cleveland Clinic Wellness Institute. "Years later, when we are once again exposed to that virus or a similar one, our antibodies become a powerful tool to prevent re-infection, either entirely or partially."

Better stamina

Better stamina

If you're thinking about signing up for a race, do it! Many of the world's top female marathoners have secured big wins during their thirties, falling right in line with research showing that our bodies reach peak fitness around age 35. "As women, we have an ability to maintain a very efficient use of our body fuel, especially our body fat," says Pamela Peeke, MD, author of Body For Life For Women. But if you're not an elite athlete, you can absolutely still slug it out into your forties, say experts. In fact, one recent study found that among recreational runners, marathon and half-marathon times were nearly identical for the age groups from 20 to 49. "So much of it is mental," says Christiane Northrup, MD, author of Women's Bodies, Women's Wisdom. "Most of the time, in your early twenties, your life has not tested you, so you don't really know what your reserves are. But now you can tap those deep, inner resources to keep going."